I know anything can be done with time and money. Is it feasible to put a automatic overdrive transmission from a 1987 F150 into a Mustang? The truck has a 5.0 in it. The car has a 289 with a 3 speed manual right now.

 

it is quite common...in fact I have one in my 65.
A little search using the word 'AOD' should net you hours of reading enjoyment.

 

Is there any special modifications you had to do, other than the normal overdrive transmission swap issues?

 

no special mods.

1 - AOD
1 - Cross member
1 - flewplate rebalanced to match the imbalance on your front balancer (or new front balancer to match the imbalance on your new flex plate)
1 - tv cable
1 - yoke
1 - possibly shortened driveshaft
1 - neutral safety harness (or make your own)
1 - shifter linkage (or make your own)
1 - Weekend

 

Theonly dumb question is when the question is not asked.The only way to learn is to ask.